Q:I never build ASM before
A: Install 11g with ASM
Q: Does anybody know if I need to install database first then Migrating to ASM?
A. Is not necesary. You should install ASM first.
Q: Do you know it's possible to build ASM on 1 raw disk?
A: yes, its possible if you pick "external redundancy" option.
" Raw devices are not available as a storage option when creating a database as a part of a new Oracle Database 11g installation. "
Yes it is possiable. In order to use ASM you need to have at least one Disk Group. In order to have one diskgroup you need to have at least one disk.
Since you have one disk for ASM , yes you can do that.If possiable use ASMLIB , which makes your effort easier.